The aim of this study to evaluate the prevalence and prognostic significance of T-wave alternans in patients with severe congestive heart failure. A number of non-invasive diagnostic studies have been developed to identify patients in this population who are at increased risk of dying suddenly. Left ventricular ejection fraction, complex ventricular arrhythmias, and neurohormonal activation have been shown in some studies to predict sudden death in patients with heart failure. The presence of beat-to-beat fluctuations of the morphology of the T-wave, referred to as T-wave alternans, has been anecdotally noted to be a harbinger of sudden cardiac death. A technique for detecting T-wave alternans at the microvolt level during exercise testing has recently been developed. T-wave alternans will be measured during bicycle and treadmill exercise tests by subjects recruited from patients who are referred for heart transplant evaluation. The study has two hypotheses: (1) patients with T-wave alternans (regardless of the etiology of their heart failure) will have an increased cardiac mortality compared with patients who do not have T-wave alternans, and (2) T-wave alternans will be significantly more common in patients with severe congestive heart failure from an ischemic cardiomyopathy than in patients with severe congestive heart failure from a non-ischemic cardiomyopathy.